Christians must pray only to God.
Scripture declares there is only 1 mediator between God and men
In the amplified bible we read the emphasis on the word only one to stress the exclusivity of the Lord Jesus Christ as spiritual heavenly mediator between God & men.’
1 Tim 2:5 For there [is only] one God, and [only] one Mediator between God and men, the Man Christ Jesus,
Although we are encouraged to pray for one another as living Christians, the concept of asking dead saints, dead relatives or any invisible, spiritual entities to pray for us crosses over an important line which is the exclusive right earned by the man Jesus Christ, the God-man who became flesh and actually earned the right to be the mediator, the high heavenly priest, the go-between that intercedes for His bride, also an exclusive relationship.
Notice the word “therefore” in Philippians 2:9 indicating an earned right was granted exclusively to Christ.
8 Being found in appearance as a man, He humbled Himself by becoming obedient to the point of death, even death on a cross.
9 For this reason also, God highly exalted Him, and bestowed on Him the name which is above every name, 10so that at the name of Jesus EVERY KNEE WILL BOW, of those who are in heaven and on earth and under the earth
See also Hebrews 1:9 where we see that 'therefore God, thy God, has anointed thee with oil of gladness ABOVE thy brethren'.
PRAYER into the spirit world belongs only to God. Mediation in the spirit realm is not simply praying for one another but once the prayers are released into the spirit world we are believing that they can BE HEARD by someone who can act on those prayers and respond. Who else can hear the simultaneous prayers around the world of millions of living men and women besides God himself?
To hear prayer, Jesus had to be, has to be, can ONLY be divine in Nature, a inseparable member of what scripture calls the “'Godhead' - See Colossians 2:9” of the one true God.
No indication of scripture indicates that any deceased person could have to ability to monitor and hear prayers. This role belongs to God alone. The living can hear and pray with us because we can contact them, the dead we cannot easily find and contact and are in fact forbidden to seek such familiar spirits.
The Key to understand here is that only because the man Jesus was already “with God and was God” according to John 1:1-2 He was God and Creator taking on a body, therefore it is legal in heaven for him to receive and mediate prayers.
Again this role of receiving, mediating and deciding how to respond to prayers is exclusive to God revealed throughout scripture as Elohim, Father, Son (from His Word, Messenger of the Lord) and and Holy Spirit.
We must no more pray to a departed saint (departed from body, but alive in the spirit) than we should pray to an Angel. If God determines to send Angels, He will, but we pray to God, not Angels.
See Don't do that! - Revelations 22:9 and again in Revelations 19:10 for confirmation and importance.
See Romans 8:26-27 where once again it is only God Himself, here specifically God working in His person as the Holy Spirit, which is able to 'make interecession for us in prayer' from the spiritual realm. - Selah
The difference between asking a living Christian brother to pray and asking a departed Christian brother, sister, or saint to pray is that one is present in the body, the other is absent from the body and now in the spirit realm present with Lord awaiting the resurrection of the body.
Finally, to seal the importance of this Christian doctrine, God the Father himself made the point emphatically at the transfiguration of Christ when Peter, James and John made the mistake ‘crossing the line’ between great the prophets Moses and Elijah and the one true mediator between God and Man which the Father sent, Jesus Christ.
See Luke 9:32-36
Luk 9:32 Peter and his companions were very sleepy, but when they became fully awake, they saw his glory and the two men standing with him.
Luk 9:33 As the men were leaving Jesus, Peter said to him, “Master, it is good for us to be here. Let us put up three shelters– one for you, one for Moses and one for Elijah.” (He did not know what he was saying.)
Luk 9:34 While he was speaking, a cloud appeared and covered them, and they were afraid as they entered the cloud.
Luk 9:35 A voice came from the cloud, saying, “This is my Son, whom I have chosen; listen to him.”
Luk 9:36 When the voice had spoken, they found that Jesus was alone. The disciples kept this to themselves and did not tell anyone at that time what they had seen.